The following disclosure is based on German Patent Application 101 01 017.6 filed on Jan. 5, 2001, which is incorporated into this application by reference.
1. Field of the Invention
The invention relates to an optical component with a low reflectance for ultraviolet light in a wavelength range between approx. 180 nm and approx. 370 nm for a high angle of incidence, in particular in a range up to at least 40xc2x0.
2. Description of the Related Art
In many areas the demand for efficient optical components with a low reflectance for ultraviolet light in a wavelength range between approx. 180 nm and approx. 370 nm is increasing. For example, light within this wavelength range is used in microlithographic systems to produce highly integrated semiconductor components or other microdevices using wafer steppers or wafer scanners, whereby a light source illuminates a mask (reticle) over an illumination optical system, which displays its image onto a semiconductor coated with a photo resist by means of a projection optical system.
Since the efficiency of this technique is defined by the exposure action speed, the aim is to use lenses having the lowest possible loss of light between the light source and the wafer. In so doing, the surfaces of transparent optical elements of the systems are coated with so-called reflection reduction layers, or antireflective layers (AR-layers) to increase their transparency or transmission, respectively. Such antireflective coatings lead to increased transmission as long as the loss of light through absorption and scattering, for example, remains smaller compared to the magnitude of the decrease in reflectance. The decrease in reflectance also serves as prevention against stray light or diffused light that can destroy the imaging properties of high-grade lenses.
As is generally known, multilayer systems with multiple stacked layers made from dielectric materials of varying refraction indices are used that usually have layers of a high refractive material and layers of thereto relatively low refractive material stacked on top of each other.
Particularly in optical systems with a multiplicity of curved lens surfaces and/or such systems in which high optical angles of incidence appear on refractive surfaces due to the high numeric aperture, it is advantageous if the antireflective coating has a homogenous effect on the entire coated surface and for all optical angles of incidence that appear, that typically lie in the range between vertical incidence and an angle of incidence of at least approx. 40xc2x0.
Often it is practical for an optical component to be transparent within the ultraviolet range not only in the active wavelength range, which is determined by the type of light source, but also in a wavelength range of visible light, for example, between approx. 500 nm and approx. 700 nm. Synchronous antireflection alleviates mask and substrate alignment for the visible spectrum as well. Hexe2x80x94Ne-laser systems with an active wavelength of 633 nm are often used for this purpose.
A two-range antireflective coating for optical elements of a processing laser is shown in German reference DE 198 31 392. The multilayer system designed for an active wavelength of 248 nm consists of nine individual layers, which interchangeably comprise magnesium fluoride as the low reflective material and aluminum oxide as the high refractive material. The first layer adjacent to the substrate in all embodiments consists of magnesium fluoride and the thickness of the magnesium fluoride layer varies inconsistently between approx. 25 nm and approx. 150 nm. These layers are supposed to have good laser resistance. After several years of use, however, fracturing in connection with delamination was observed in magnesium fluoride/aluminum oxide multi layer systems with similar layer arrangement and layer thickness distribution, causing the life spans of such coatings and optical components coated therewith to be limited.
An object of the invention is to provide an antireflective coating for optical components that offers effective antireflection for ultraviolet light in the wavelength range between approx. 180 nm and approx. 370 nm and high angle of incidence ranges up to at least approx. 40xc2x0, and that is distinguished by high resistance, in particular under laser beams.
To solve this and other objects, the invention, according to one formulation, provides an optical component with a low reflectance for ultraviolet light in a wavelength range between approx. 180 nm and approx. 370 nm for a high angles of incidence, which includes:
a substrate having at least one surface;
a multilayer system of multiple stacked layers arranged to the at least one surface of said substrate to provide a reduction of reflection, a layer having a high refractive or a low refractive dielectric material;
a first layer contacting the substrate being substantially free of magnesium fluoride and none of the layers having a layer thickness of more than about (0.5xcex), where xcex is the working wavelength in the ultraviolet wavelength range.
An antireflective multilayer system according to the invention distinguishes itself in that the first layer adjacent to the substrate is substantially free of magnesium fluoride and that none of the layers has a thickness of more than approx. (0.5xcex), whereby xcex is the working wavelength in the ultraviolet wavelength range. Therefore, the first layer should neither contain magnesium fluoride nor consist of this material respectively. In any event, any amount of magnesium fluoride present should be so low that the properties of this first layer are not substantially affected by the properties of magnesium fluoride. It is particularly favorable if none of the layers has a layer thickness of more than approx. (0.35xcex). The layer thicknesses of the low refractive material have proven to be particularly critical, whereby said material is preferably a fluoride, in particular magnesium fluoride. The layer thickness should not exceed approx. 0.3xcex in any of the layers.
The invention and its other advantageous embodiments show that to produce permanently laser-resistant antireflective coatings with the optical properties mentioned initially, it is necessary to choose from theoretically infinite number of optically appropriate multilayer systems those systems that fulfill the given boundary conditions in terms of material selection for the layer adjacent to the substrate and in terms of maximum layer thicknesses. Because of its low refractive index and good workability, commonly used magnesium fluoride, in particular, could be identified as a critical material, which contrary to traditional suggestions should not be used as the first layer adjacent to the substrate. Furthermore, layer thicknesses of this material should be kept as low as possible. As a result, magnesium fluoride has been identified as the xe2x80x9coperative weak linkxe2x80x9d of such coatings. Its negative influence on the resistibility of the antireflective coating can be minimized by means of appropriately thin individual layer thicknesses and lowest possible overall layer thicknesses.
It has turned out that particularly high laser resistance and other types of resistance, for example, resistance towards layer stress that leads to fracturing if the overall layer thickness of the multilayer system totals less than double the active wavelength, in particular less than 1.5xcex. The total thickness of the low refractive material layers can be less than the active wavelength and in particular less than half of the active wavelength.
Within the context of the invention it is possible to build multilayer systems that have no more than six individual layers. Because of the fewer number of layers as compared to commonly known multilayer systems of this type, the laser durability of the coating can normally only be improved such that the probability of defects leading to degradation within the multiplicity of layers is less, the fewer the layers that need to be arranged. Such flaws that minimize laser durability may be impurities, defects or enclosures that increase local absorption and can lead to uneven beam exposure of the layer. Reducing the number of layers also simplifies the process, which can decrease costs for supplying coated optical elements.
It has become furthermore evident that it is advantageous if the ratio between the sum of the layer thicknesses of the low refractive material layers and the sum of the layer thicknesses of the high refractive material layers lies below approx. 1.5, in particular below approx. 1.2. This may well positively counteract the buildup of thermal stress within the layer system and between the layer system and substrate, as well as inner layer stress.
Beyond the claims, the aforementioned and further characteristics emanate from the description and the drawings as well, whereby the specific characteristics may be realized individually or in groups in the form of sub-combinations in an embodiment of the invention and in other areas and may represent embodiments that are favorable and independently protectable.
Example embodiments of the invention are illustrated in the drawings and explained in more detail below.